The present invention relates to a method for the preparation of a polyvinyl chloride resin by the polymerization of vinyl chloride monomer dispersed in an aqueous polymerization medium or, more particularly, to a method for the preparation of a polyvinyl chloride resin powder having excellent flowability.
It is a conventional process in the prior art that a vinyl chloride-based polymeric resin is prepared by dispersing and polymerizing vinyl chloride monomer or a monomer mixture mainly composed of vinyl chloride monomer in an aqueous polymerization medium containing a dispersing agent, polymerization initiator and the like to produce a polymerizate slurry which is then dehydrated and dried into a resin powder.
One of the serious drawbacks frequently encountered in the above mentioned process is that the resin powder of polyvinyl chloride is charged with static electricity in the course of drying or during transportation to decrease flowability sometimes causing heavy blockage of the drying oven or pneumatic pipe line for transportation. In addition, the bulk density of a resin powder is remarkably decreased when the powder is electrostatically charged to cause an error in the volumetric measurement of the amount in place of the measurement by weight. This drawback due to electrostatic charging of the resin powder is more serious in winter of low temperature and low humidity than in summer.
Various proposals and attempts of course have been made in the prior art to solve the problem due to the electrostatic charging of a polyvinyl chloride resin powder. A conventional method most widely undertaken is to admix the resin powder with a surface active agent, which may be non-ionic, anionic or cationic, so as to impart the surface of the resin particles with increased water-retentivity or hydrophilicity. This method, however, is not always quite effective for the purpose of preventing decrease in the bulk density or flowability of the resin powder and, in addition, has a disadvantage that the thermal stability of the resin is greatly decreased.